Electric lamp for producing short wave length radiation



Feb. 2, 1937. c. H. BRASELTON 2,069,713

ELECTRIC LAMP FOR PRODUCING SHORT WAVE LENGTH RADIATION Filed Sept. 25, 1932 Jagj INVENTOR Patented Feb. 2, 1937 ELECTRIC LAMP FOR PRODUCING SHORT WAVE LENGTH RADIATION Chester H. Braselton, New York, N. Y., assignor to Sirian Lamp Company, Newark, NHL, a corporation of Delaware Application September 23, 1932, Serial No. 634,481

3 Claims.

This invention relates to radiation devices for producing short wave length radiation and is dlrected particularly to an electric lamp having a filament for producing radiation at the violet end of the spectrum including ultra-violet invisible rays.

The principal object of the invention is to provide an electric lamp with a high output of ultraviolet radiation.

Another object of the invention is to provide a filament for an electric lamp which when raised to its operating temperature will be rich in ultra-violet radiation.

Another object of the invention is to maintain in a lamp a radiating surface at a temperature near the melting point of the material forming such surface so as to increase its radiation of' short waves both visible and invisible.

Another object of the invention is to provide a lamp filament of metal having a radiating surface of another metal which has a lower melting point.

Still anotherobject of the invention is to pro vide a method of applying a radiating surface to a filament.

Other objects of the invention will be apparent as the description proceeds.

The amount of short wave radiation in the neighborhood of the violet end of the spectrum and including radiation outside of the visible spectrum itself appears to increase from a radiating surface as the temperature of the surface approaches its melting point. This invention contemplates, therefore, providing means to maintain in an electric lamp a radiating surface at a. temperature closely approaching its melting point by mounting the surface on a base of a metal having a. higher melting point which can stand the temperature under which the device is operated. Y 7

While a large number of metals may be selected for developing this principle to come within the province of. this invention, I may prefer to use a. tungsten filament upon which has been applied a surface coating -of tantalum. Tungsten has a melting point of about 3200C. While the melting point of tantalum is in the neighborhood of 2850 C., and hence the temperature of the filament may be maintained to a point almost reaching the melting point of tantalum without affecting the strength of the tungsten core. Thus with the tantalum maintained near its melting point the short wave radiation therefrom is greatly increased.

The invention is illustrated in the accompanying drawing, in which:

Fig. -1 is a sectional elevational view of a lamp which may be the preferred form of the invention;

Fig. 2 is an enlarged longitudinal sectional view of the coiled filament of the lamp of Fig. 1; and

Fig. 3 is an enlarged fragmentary view partly in section of a slug out of which the lamp filament'may be formed.

The lamp shown in Fig. 1 may comprise an en'- velope I of a material which is'transparent to the ultra-violet rays, such as quartz, uviol", vita glass, corex, sanalux", sun-ray, and the like. This envelope may be shaped similarly to an ordinary electric light bulb and may be provided with a neck II to the end of. which is fused the inturned press I2. Two filament support rods I3 and-I4, are sealed'in the press and extend upwardly to a point near the center of the rounded portion of the envelope and to the ends thereof are attached the ends of the closely coiled filament I5, as indicated. A central glass stem I6 may be fused to the press I2 and may extend upwardly along the axis of the envelope to a point adjacent the upper end of the rods I3 and I4 where it may terminate in a bead H which carries a plurality of fine support wires I 8 for supporting the middle part of. the filament which is preferably bent to form a portion of a circle. v

The envelope may be filled with a suitable inert gasfor increasing the heat radiation from the filament. This gas may be a mixture of about 84% argon and 16% nitrogen at a pressure such that at the operating temperature the pressure will be about one atmosphere. A standard type of screw base I9 may be mounted on the end of the neck II with the usual cement 20, and the side of the base 2i may be attached by a leadin wire 22 to the support rod I3; while a second lead-in wire 23 may connect the rod I4 with the center contact terminal 24 as indicated.

The filament I shown enlarged in Figure 2 of the drawing, is preferably formed of a. tungsten core 25 provided with a. thin surface coating 26 of tantalum. One way of applying the tantalum coating to the coiled tungsten filament is to mix finely divided tantalum pentachloride with alcohol until a paste is formed which can then be applied to the surface of the wire. This paste may be brushed onto the filament, or the mixture of alcohol and tantalum pentachloride may be made thinner and applied to the filament by tains a shiny white surface which appears to be tantalum metal but may and probably does contain tantalum hydride. But whatever the constituentsof the coating, it has a melting point lower than that of tungsten, and if this filament is-mounted in a lamp, preferably with an inert gas, as mentioned above, the operating temperature can be maintained at a point close to the melting point of tantalum. In the presence of an inert gas the temperature may be maintained higher without danger of damaging the filament as the gas tends to rapidly dissipate the heat evolved. The filament may, however, be operated in a vacuum although the results obtained are not as good.

In some cases it may be preferable to apply the tantalum coating before the filament is drawn into a wire. In this case a slug 21 (Fig. 3) may be coated with tantalum in a. manner described above for the filament. After the tantalum coating has been sintered onto the slug by heating, the slug may be swaged to decrease its diameter and compact the metal particles more strongly together, after which the slug may be drawn into a wire similar to the manner of drawing tungsten wire. .After the wire is formed it has a. surface of a thin film of the tantalum and may be coiled or otherwise formed into the proper shape of the filament.

Other methods of applying. the metal to the filament core or to theslug before drawing may be'used, such as by electro-plating or spraying molten metal thereon, it being only necessary that the coating metal be firmly adherent to the core or otherwise arranged so as not to be thrown off therefrom.

When inthe preferred form of the invention a tungsten core is coated with a coating of tantalum the tantalumwhas another beneficial effect as the coating prevents the evaporation-of the tungsten core caused by the presence of a slight amount of water vapor in the bulb. Water vapor attacking the tantalum coating appears to form a stable tantalum oxide which is not affected by tungsten forms tungstic oxide and free hydrogen. The oxide is deposited on the wall of the lamp where hydrogen combines with it again to form free tungsten and water vapor, and the cycle is repeated until the filament is destroyed. The

coating, therefore, acts as a protecting shield for the tungsten core and also as a clean-up agent to eliminate any water vapor which may be present.

While the complete coating of the lower melting point metal upon the core may be preferred the invention also contemplates the incorporation of such lower melting point metal with the other metal to form a physical mixture or an alloy. The invention also contemplates the association of the lower melting point metal with that of higher melting point without any physical attachment therebetween, as by merely positioning the two in close juxtaposition or by wrapping a wire or ribbon of the lower melting point metal around a wire of thehigher melting point metal. Such a construction, however, will not be as effective as the coating because the latter provides a larger radiating surface and may be maintained at a higher temperature due to the rapid heat flow between the core and the coating.

Many other variations and changes in the construction shown and described may be resorted to without departing from the spirit of the invention, and I do not, therefore, desire to limit myself to what has been shown and described except as such limitations occur in the appended claims.

What I claim is:

1. An electric lamp comprising an envelope having a high degreeof transparency for ultraviolet radiation, a tungsten filament within said envelope, a coating of tantalum on the surface thereof and closely adherent thereto, and an inert gas including argon and nitrogen within said envelope, the pressure of said gas being approximately atmospheric.

2. An electric lamp comprising an envelope having a high degree of transparency for ultraviolet radiation, a tungsten filament within said envelope, a coating of tantalum on the surface thereof and closely adherent thereto, and an inert gas by volume over of argon and over 10% of nitrogen within said envelope having a pressure substantially atmospheric.

3. An electric lamp comprising an envelope having'a high degree of transparency for ultraviolet radiation, a tungsten filament within said envelope, and a coating of tantalum and tantalum hydride on the surface thereof and closely adherent thereto.

CHESTER H. BRIASELTON. 

